There is no "tax break" for buying a ring. Your boyfriend is apparently very gullible.
His trip to the jewelry store:
BF: "I'm looking for a modestly priced engagement ring."
Salesperson: "Well, today is your lucky day! Just this weekend we have a special deal, and it's part of the government's stimulus plan. If you buy this $20,000 ring, you can get a tax credit of $25,000. In essence, you get the ring for free AND get money back!"
BF: "That doesn't sound right... how does this work?"
Salesperson: "The government is trying to stimulate the economy, and the best way to do that is to encourage the purchase of very expensive items. But it's only for this weekend, so you have to act now!"
BF: "Sounds good. I'll take it!"
This is not an FML for you. It will become an FML if you still decide to marry him, knowing that he is a moron who thinks he can get a "tax break" for buying jewelry.

Even if he gets a tax credit (how?), that credit will not exceed the money he spent on the ring. Should be pretty obvious, right? OP is a whiner, of course he bought the ring because he wanted to marry her, he is just trying to keep his finances in order. Hopefully he'll change his mind now.